Locking up freed immigration detainees under terrorism protection laws has been floated after a High Court ruling.
Some 93 ex-detainees - including three murderers and several sex offenders - are being fitted with ankle monitors and face imprisonment if they breach tight reporting requirements after being released from detention in recent days.
The High Court is yet to release the full reasons for its landmark ruling that holding people indefinitely in immigration detention was unconstitutional.
The finding was based on the premise that indefinite detention was a form of punishment and only the courts had the power to discipline criminals, not the minister.
Emergency legislation to enforce strict monitoring and reporting rules for the freed detainees passed federal parliament on Thursday.
Opposition home affairs spokesman James Paterson said the government had been caught flat-footed and more should have been done to keep the serious offenders behind bars.
Police can apply to the courts to keep terrorists who have completed their sentences locked up if they pose a risk to the community, to have them detained before being convicted and to heavily supervise them once they're released.
Senator Paterson said the government should have considered such a regime.
The Liberal senator said it was "a well-accepted and well-tested area of law that could be applied at the very least to the highest risk offenders".
"The government should be looking at this as an option ... some are very serious, violent offenders," he told the ABC's Insiders program on Sunday.
"The government was not ready and did not do that. Those could have been used to detain, at the very least, the highest risk among this cohort."
Senator Paterson said he expected the new monitoring regime would apply to almost all of the released offenders, despite potential legal questions over ministerial discretion after the ruling.
Home Affairs Minister Clare O'Neil said further actions would follow once the reasoning became clear so the government could be sure any new regime could withstand another legal challenge.
"When the reasons for the decision are handed down at the beginning of next year, we will then be able to look at what else we can do," she told Sky News.
The new scheme was the best way to ensure community safety while acting within the confines of the law, she said.
Ms O'Neil has argued she would have kept the offenders in immigration detention indefinitely if she had the power to do so.
She also admitted her department had received advice they were likely to win the court case but added possible responses to any ruling had been considered.
The 93 people are part of a broader 340-strong cohort that has been in immigration detention for more than a year.
It was unlikely the High Court ruling would apply to all of them, the minister said.